Sensors and measuring means for measuring and weighing vehicles in motion (weigh in motion—WIM) are generally known. A tubular sensor is already described in EP-A-0491655. FIG. 1 shows a hollow profile sensor according to U.S. Pat. No. 5,461,924, which discloses another hollow profile sensor with a tube section that contains a measuring arrangement in its profile interior, wherein said measuring arrangement is non-positively connected to the wall of the tube section. The tube section features a force introduction flange that is connected to the tube section flanking thereto in such a way that the force application lines are concentrated on the measuring arrangement and a mechanical amplifier effect is achieved. The lines of force P indicated by the arrows in FIG. 1 generate a concentrated force in the hollow profile sensor as indicated by a double arrow that centrally extends through the measuring element illustrated in the center.
In other known embodiments, the force of the wheels of the vehicles driving over the hollow profile sensor is not introduced into the measuring element in a concentrated fashion, but rather extends from the respective location into the ground relatively vertical or partially via lateral supports, wherein only one component of the force respectively extends through the measuring element. Such embodiments proved insufficient with respect to the nowadays required accuracy for the aforementioned applications. In addition, most older applications do not make it possible to measure shearing forces.
In contrast to other known embodiments that feature elastomers between the measuring elements and the force application in order to absorb transverse forces, it proved advantageous with respect to the accuracy if the flow of forces is transmitted from the vehicle to a wide force introduction flange by means of a hard surface, wherein said force introduction flange in turn transmits the forces directly to the measuring element in a concentrated fashion. In this case, no elastic materials at all can be arranged between the vehicle wheel and the measuring element and essentially all lines of force P should extend through the measuring element in order to prevent force shunts.
In the aforementioned arrangement, the sensor consists of a quartz crystal sensor with stable electrical and mechanical properties that is decoupled from lateral forces and has no interfering signal drift.
Although hollow profile sensors of this type function reliably over long periods of time, their manufacture and installation are very elaborate due to the closed one-piece design of the hollow profile. An industrial manufacture, in particular, in large quantities therefore could hardly be realized and would not be economical.
Piezoelectric measuring elements need to be installed in a sensor structure under prestress. In the prior art, the prestress is generated in that the sensor elements are manufactured with a cover and subsequently installed in the hollow profile.
It is impossible to adjust and control the local prestressing force because it is dependent on different factors such as, e.g., the height, flatness and parallelism of the contact surfaces over the entire length of the profile, the height of the measuring arrangement, the cross-sectional geometry, in particular, of the tube section, as well as the modulus of elasticity of the profile material (e.g., aluminum, steel or other alloys). In a closed hollow profile, a uniform prestress over the length thereof can be achieved if the wall thickness is realized in a highly accurate uniform fashion. As the length increases, however, it becomes nearly impossible to process the contact surfaces in the interior, between which the measuring elements are arranged, with uniform accuracy over the entire length of the profile. Even a slight inaccuracy of these geometries already leads to different prestresses in the individual measuring elements and therefore to inaccurate measurement results.